DNA supercoiling has been quantitatively studied with single-molecule techniques[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}. The extension-twist curves from experiments reveal a coupling between supercoiling (linking number) and DNA extension under stretching force[1]}, [2]}, [3]}. The discontinuities on torque and extension versus linking number were also observed in experiment[14]}. Experiments with a magnetic tweezers pulling a fluorescently labeled DNA have directly imaged the coexistence phase, and remarkably, measured the time-dependent supercoiling dynamics, e.g., plectoneme diffusion, nucleation and hopping[9]}. The propagation of plectoneme along DNA via diffusion presents conformational rearrangement of DNA at about hundreds bps per second. The hopping happens comparatively fast and a fastest one observed takes tens of milliseconds for a displacement of thousands of bps[9]}. The surviving time of the individual plectoneme from nucleation to vanishing, i.e., the plectoneme life-time, spans from milliseconds to several seconds.
